I like bouchers for working with OTTBs because they encourage young horses to come into the bit and collect up, which is great for them to learn early on. Your problem, though, is not in equipment--it's in the horse's training. Nothing but time can train balance and form into a horse.
I advice against a curb bit, it will make it that much harder to transition to a snaffle bit when it comes time. Kimberwickes, tom thumbs....they're not what you want to use in an English horse because of the reliance your horse will have upon the curb action. Your goal is to be able to ride this horse in a simple jointed snaffle, or similar. I would put him in a boucher or an eggbutt/Dee ring and work with him there.
My trainer, when she got Molly back (she trained her off the track, sold her to people who messed her up, sold her to someone else who quickly got scared of her and sold her to me....I found her off track trainer and had her retrain her) is very experienced, since Molly had begun bolting as a result of bad riding by her second off-track owner, she put her in a Pelham so she could retrain her. After a few weeks, the pelham was gone and she was over the worst, so she was able to go back to work with her in a boucher. If done wrong, though, a Pelham can do more harm than good.
A trainer would help you out a lot with this, especially if you're new to OTTB's. Are there any reputable English trainers in your area you guys could take lessons from?